Vehicle emission control systems may include selective catalytic reduction (SCR) systems to reduce NOx emissions. SCR systems involve injection of a reductant upstream of an SCR catalyst. The reductant, or reductant products, then reacts with exhaust NOx species to create byproducts such as nitrogen and water. In SCR systems, variations in temperature may cause potential issues with both the conversion efficiency of the catalyst as well as the amount of reductant stored on the catalyst. For example, as the temperature of the catalyst increases beyond a threshold, reductant may be desorbed from the SCR catalyst resulting in ammonia slip.
Emission control systems may also include particulate filters for removing particulates from exhaust gas. During filter regeneration, heated exhaust gas may be passed through the filter to raise the filter temperature above the normal operating temperature and burn off particulate matter previously stored. Consequently, various issues may arise in emission control systems including SCR catalysts and particulate filters. For example, temperature regulation in such systems may be complicated. On one hand, the exhaust temperature may need to be maintained within a desired range, for example, the operative range of the SCR catalyst. On the other hand, the exhaust temperature may need to be periodically raised to enable filter regeneration without degrading catalyst performance.
In one example, the above mentioned issues may be addressed by a method for operating an engine including an SCR catalyst downstream of an exhaust turbine and a particulate filter upstream of the turbine. In one embodiment, the method comprises, adjusting a turbine wastegate based on at least one of a catalyst temperature and an operating mode of the filter.
By positioning the filter upstream of the turbine, and the catalyst downstream of the turbine, the temperature of the SCR catalyst may be maintained even while the filter is being regenerated. By adjusting a turbine wastegate responsive to filter operation, the temperature of the SCR catalyst may be controlled. Thus, when the filter is storing particulates, flow through the wastegate may be increased to keep the catalyst temperature in the operative range. Then, when the filter is being regenerated, flow through the wastegate may be decreased to reduce the amount of hot exhaust that is routed through the catalyst. The injection of an SCR reductant upstream of the turbine may also be adjusted responsive to the wastegate adjustment. In this way, the temperature of an SCR catalyst may be controlled and coordination between the various emission control devices may also be achieved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.